A control unit for an air conditioner employed in a vehicle or the like controls driving of an air outlet switching door by using a DC motor in order to control opening and closing of the air outlet. When opening and closing of the air outlet switching door is controlled, in order to set a degree of opening and closing precisely, an origin position of the DC motor is detected first and then the opening and closing is controlled in reference to the origin position. A technique used to this end is as follows. That is, an origin position pattern is detected, for example, by rotating the DC motor in a predetermined direction within a rotatable range of the DC motor or the air outlet switching door. In a case where a rotational position of the DC motor reaches an end of an operable range without detecting the origin position pattern, the origin position pattern is detected again by inverting the rotational direction.
Incidentally, as a method of controlling a plurality of DC motors in the air conditioner described as above, for example, two DC motors are driven by three half-bridge circuits. A configuration in this case is to control energization by connecting terminals of the two DC motors at one end to a common half-bridge circuit and connecting terminals at the other end to different half-bridge circuits.
According to this configuration, driving of two DC motors connected to three half-bridge circuits cannot be controlled in a completely independent manner. In a case where one DC motor is driven to rotate while the other motor is also driven, the one DC motor is not allowed to change the rotational direction and it is necessary to wait for the driving of the other DC motor to stop. In this case, one of the DC motors whichever is responsible for operations more readily recognizable by a user is operated preferentially so that the user does not feel uncomfortable.
However, when the configuration as above is adopted, of the origin position pattern detection times by the two DC motors, the longer time is dominant in an operation to detect the origin position pattern described above. Hence, a contribution to shortening of the detection time depends on whether this time can be shortened or not. However, in the opening and closing control of the doors of the air conditioner, a priority is assigned to the detection operation by one of the two DC motors connected to the three half-bridge circuits whichever is to be operated preferentially. Hence, a detection time of the DC motor on a non-priority side requires a further longer time in some cases.